1. Field of the Invention
This invention relates to a method of making a magnetic recording medium by forming a thin magnetic film on a moving flexible substrate made of a high-molecular material or the like by vacuum deposition.
2. Description of the Prior Art
Coating type magnetic recording media are widely used. These magnetic recording media use powdered magnetic materials such as magnetic oxide particles and ferromagnetic particles, for example, .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped Fe.sub.3 O.sub.4, Berthollide compounds of .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, CrO.sub.2 or the like. These powdered magnetic materials are dispersed in organic binders such as vinyl chloride-vinyl acetate copolymers, styrene-butadiene copolymers, epoxy resins and polyurethane resins. The dispersions thus obtained are then applied in coats on non-magnetic substrates and dried to form the magnetic recording media. Recently, so-called non-binder type magnetic recording media using no binders have attracted attention because of their ability to meet strong demand for high density recording. The magnetic recording media of this type have magnetic recording layers which consist of thin ferromagnetic metal films formed by a vapor deposition process such as vacuum deposition, sputtering or ion plating, or a plating process such as electroplating or electroless plating. Thus various efforts are being made to develop non-binder type magnetic recording media suitable for practical use.
In the conventional coating type magnetic recording media, metal oxides exhibiting saturation magnetization lower than that of ferromagnetic metals are mainly used as the magnetic material. If the thickness of the magnetic material layer on the substrate is reduced, the signal output level also drops because of low saturation magnetization. Therefore, it is impossible to reduce the thickness of the magnetic material layer to such an extent as required for the high density recording. In addition, the coating type magnetic recording media are also disadvantageous in that they require complicated production processes and bulky equipment for the recovery of solvents or for pollution control. On the other hand, the non-binder type magnetic recording media can be provided with extremely thin magnetic films suitable for high density recording. In this type of magnetic recording media, ferromagnetic metals exhibiting higher saturation magnetization than metal oxide magnetic materials can be formed as extremely thin films on substrates because non-magnetic materials such as binders are not used and, in addition, the process for preparing the recording media is simple.
It is theoretically and experimentally suggested that, in order to achieve high density recording, the magnetic recording media should exhibit a high coercive force and a smaller thickness. In view of this, much hope is placed on the non-binder type magnetic recording media which can be formed to a far smaller thickness and exhibit a higher saturation magnetic flux density than the coating type magnetic recording media.
Particularly, the vacuum deposition process is very advantageous because, unlike the plating process, it requires no waste water treatment, the production process is simple and the film deposition rate is high. To make a magnetic film exhibiting a coercive force and a squareness suitable for magnetic recording media by the vacuum deposition process, it has been proposed in U.S. Pat. Nos. 3342632 and 3342633 to deposit a vapor flow on a substrate at an oblique angle with respect to the substrate surface. With this method, the larger the incident angle of the vapor flow impinging upon the substrate surface, the higher is the coercive force of the obtained medium. However, increasing the incident angle of the vapor flow results in a lower deposition efficiency and presents a problem with regard to production efficiency.
To form a thin magnetic film exhibiting a high coercive force by depositing it at a relatively small incident angle with respect to the substrate surface, it has been proposed to introduce oxygen into the vacuum tank during the oblique incidence deposition. However, this conventional method yields magnetic recording media exhibiting insufficient reproduction output, high noise and low adhesion between the magnetic film and the substrate.